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A method for strengthening the bond between cement stone and aggregates
CEMENT and CONCRETE RESEARCH. Vol. 17, pp. 651-660, 1987. Printed in the USA. 0008-8846/87 $3.00+00. Copyright (c) 1987 Pergamon Journals, Ltd.
A METHOD FOR STRENGTHENING THE BOND BETWEEN CEMENT STONE AND AGGREGATES R. Zimbelmann U n i v e r s i t ~ t der Bundeswehr MUnchen Neubiberg, F. R. Germany
(Communicated by F.W. Locher) (Received March 25, 1987) ABSTRACT On the basis of the knowledge about the contact zone between aggregate and cement stone researches were c a r r i e d out with the aim to enlarge the bond strength of these components. The researches lead to an increase of bond strength to such an a l t i t u d e , t h a t at the age of one year the t e n s i l e strength of cement stone was exceeded. For t h i s purpose the aggregate surface had been covered by a suspension of pumice, detergent and water glass. In pull out t e s ~ w i t h steel bars, which were covered with the suspension described above a remarcable increase of bond i n concrete was obtained. Introduction In (1) the author described the contact zone between aggregate surface and cement stone. According to t h i s cement stone does not surround the aggregate grains in concrete in a homogenous mass, but a system of d i f f e r e n t layers i s formed. The contact zone, cons i s t i n g of the contact layer and the intermediate l a y e r , to a predominant part consists of calcium hydroxide (Ca(OH)2). By t h i s material also the connexion (bond) of cement stone to the aggregate is set up. This area does represent the weakest zone i n concrete s t r u c t u r e exposed to compression, tens:ion or shear stresses. In f i g u r e 1 the d e s t r i b u t i o n of stresses arround the aggregate i n concrete loaded by compression is shown.
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
/i:i!i:.;:11;i;!;!i!i:i!;:.i!;2:::i:'::.:i:i::::.::. (cement stone ]
i!iii!!ii
!iiiiiii!;)iiiiiiii!iii!?iiiiiiiiiiiiiii
Figure 1 : D i s t r i b u t i o n of stresses arround a compression loaded aggregate 651
652
Vol. 17, No. 4 R. Zimbelmann
An increase of the strength properties of concrete therefore should to be reached by strengthening the contact zone, whatever one could understand of "strengthening". The following t r e a t i s e shall describe the thoughts and experiments which led to a remarkable increase in bond strength between cement stone and aggregate. Formation of the contact zone The basis for the researches on the modification and strengthening of the contact zone had to be the knowledge upon the reason of i t s f o r mation, expecially the concentration of the Ca(OH)2 and the high porosity of this zone, reaching up to a percentage of cavity of about 5o %. These blo symptoms are a sign of a r e l a t i v e l y hlgh water content existing in this area during and a f t e r bringing toaether the components aggregate and cement mortar. This supposition also is sustained by confirming that no cement grains could be found in the area of the contact zone in a thickness of about 15 to 25 um. This led to the conclusion that during mixing and bringing cement mortar on the aggregate a water f i l m is formed arround each solid p a r t i c l e . The thickness of the water film may be about half of the thickness of the contact zone, i t thereafter may reach about io pm. This is schematically shown in figure 2. !
ticles
Figure 2: Formation of the water film during bringing together cement mortar and aggregate
I
wnfer film
i.n
oggregate After bringing water to the solid p a r t i c l e s , these are covered at once by the water f i l m , t~hich posesses nearly a constant thickness at a l l p a r t i c l e s . In consequence of this water film the p a r t i c l e s are separated from each other at a distance of about twice of the thickness of water film. At the beginning of hydration of the cement calciumhydroxide and the components, which l a t e r w i l l form e t t r i n g i t e are dissolved in the mixing water arround the cement grains and diffuse into the vlater f i l m arround
Vol. 17, No. 4
653 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
the i n e r t aggregate. Here at the i n e r t aggregate surface the dissolved substances can c r i s t a l l y s e in a qreater extent than at the periphery of the cement grains, which permanently is changing. The contact layer is formed. I t can be presumed that by the accumulation of crystals at the aggregate surface and the growth of the crystals the water film may be expelled and thereby at least a part of the expelled water may extent the distance from the aggregate surface to the nearest situated cement grains. During hydration a dense gel is formed arround and between the cement grains, which is growing from both sides of the adjacent cement grains. At the cement grains adjoining the i n e r t aggregate the hydrates only emanate from one of the two partners, the cement grain. The water film arround the aggregate therefore is bridged by some great crystals finding here much space for growing. Thougths about strengthening the adhesion Decisive for the formation of the contact zone and therefore for the low bond strength the two following factors may be seen: a) formation of a water film arround the aggregate b) c r i s t a l l i s a t i o n of calcium hydroxide in form of the contact layer. Starting from this thought t h e o r e t i c a l l y there can be expected an augmentation of bond in consequence of the following measures. F i r s t p o s s i b i l i t y : By using tensides the surface tension of water is reduced. The'consequence out of this is a diminishing of the water f i l m , thereby a greater density of the contact zone and an enlarging number of contact points between the contact layer and the intermediate layer may be reached. So the area to transmit the stresses could be enlarged. Second p o s s i b i l i t y : I f there could be induced a chemical or a physical reaction between the aggregate and the cement stone - as i t can be supposed of the lime stone aggregate - a t i g h t connection over the aggregate surface could be reached. Third p o s s i b i l i t y : A combination of the p o s s i b i l i t i e s 1 and 2 could strengthen the effect of bonding because of the diminished water film out of the use of a tenside the partners of the reaction can move closer. Fourth p o s s i b i l i t y : I f one deposits reactive substances at the aggregate surface i t might be possible to receive reaction products out of this substance and the calciumhydroxid, which have a greater physical a f f i n i t y to the aggregate - in most of the cases to q u a r t z i t i c aggregates - and at the same time gain a greater density of the contact zone. The reactive substance would then be a bridge between the aggregate surface and the cement stone. Experiments about the strengthenin~ of the adhesion In a short test the f i r s t variant of the 4 p o s s i b i l i t i e s was tested. In an extensive research the fourth variant was examined. In the tests with a tenside, a small quantity of tenside was added to the mixing water. The tenside consisted of a washing agent of normal commercial usage. The water with the added agent (about 1 to 3 % of cement weight) was slowly mixed together with a portland cement PZ 35 F
654
Vol. 17, No. 4 R. Zimbelmann
in a laboratory mixer so that no foam was formed. The water-cementr a t i o by weight was o.35. The exposition auf the cement on the aggregate and the performance of the tests were carried out as described in ( I ) . The t e s t i n g of the bond at only a few samples of polished lime stone, quarz gravel and polished feldspar showed an increase of the bond strength of about Ioo to 18o per cent in r e l a t i o n to the values of bond strength reached with the same aggregates and the same cement but without the use of a tenside. The r e s u l t s of these f i r s t tests are shown in figure 3.
3,0 2,8 2,6 ~E
(28o%) I
2,4
E 2,2
z 2,o .~ 1,8r-
I
1,6
cm 1,z,
t'--
1,2 •4.- 1,0
t
(200%)
..~ 0,8 c 0,6 o •. ~ O,L, 0,2
p-
~s
2
;
/
z limestone, polished
/
__
q ucLrtz, grave[ fe[clspar, polished
f
~...~~. ~ 2
~
6
~
81o 20 co 60 80100 time in days
Figure 3: F i r s t tests of strengthening the adhesion between cement mortar and aggregate by adding a tenside to the mixing water
An examination of the contact zone by the scanning electron microscope showed t h a t the contact zone was - as expected - s i s n i f i c a n t l y thinner and of higher densi~,. The contact layer had a thickness of about o,5 to i ~m insted of 2 to 3 um. The region of the intermediate zone showed a changed s t r u c t u r e in comparison to cement stone without tenside. Besides of the well known form of the calcium hydroxide c r i s t a l l s (small sheets and great panel shaped c r i s t a l l s ) the s t r u c t u r e was s o l i d with some greater pores. The s t r u c t u ~ , rich of pores, consisting of a texture of needle c r i s t a l l s out of e t t r i n g i t e and CSH mostly was not to be identifyed. Moreover there was remarkable t h a t in spite of the humid storage already at a r e l a t i v e l y ~oung age (28 days) the contact layer showed numerous microcracks. With increasing age needleshaped CSH-crystals formed in the pores of the intermediate layer.
Vol. 17, No. 4
655 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
As s i g h t photographs of the aggregate surface showed a f t e r the bond t e s t , the area of the contact l a y e r , which had been losened by the bond t e s t , was greater than in the t e s t w i t h o u t the use of tenside. That means, t h a t the linkage between the contact l a y e r and cement stone had been increased and to t h i s the enhancement of the bond strength may be r e l a t e d . Figure 4 shows the contact l a y e r a f t e r the bond t e s t . The numerous cracks are c l e a r l y to be seen. Figure 5 shows an area of the intermediate layer w i t h o u t the use of tenside and f i g u r e 6 with the use of tenside. d ~
I
&O~m
Figure 4: Contact layer a f t e r the use of a tenside
The t e s t according to the p o s s i b i l i t y 4 based on the f o l l o w i n g idea. I f i t was possible to replace the contact layer of calcium hydroxide by a layer of calcium s i l i c a t e s , i t could be expected, t h a t , e s p e c i a l l y in those cases the aggregate would be of quartz or would contain parts of quartz, there would e x i s t a higher physical a f f i n i t y between t h i s s i l i c a t e contact layer and the aggregate surface. Besides of t h i s a b e t t e r connection between the s i l i c a t e contact l a y e r and the hydrates of the cement could be expected. F o r t h e case t h a t t h i s s i l i c a t e layer does cons i s t of a pozzolanic m a t e r i a l , the calcium hydroxide may react with i t by forming CSH. So the formation of a l a y e r or of greater comprisals c o n s i s t i n g of calcium hydroxide might be avoided.
10 .,}]m Figure 5:Intermediate layer w i t h o u t the use o f a tenside
On the bases of these thoughts several suspensions out of waterglass (Potasium s i l i c a t e and sodium s i l i c a t e ) and
656
Vol. 17, No. 4 R. Zimbelmann
pozzolanics were mixed. The waterglass on the one side had to fasten the pozzolanic on the aggregate surface and on the other side i t had to give a greater density to the contact zone - especially to the indermediate layer by i t s products of decomposition in the reaction with the calcium hydroxide. F i r s t researches showed that the effect of such a suspension was the b e t t e r , the thinner the film out of this suspensi£n on Figure 6: Intermediate layer a f t e r the the aggregate surface use of a tenside could be formed. As pozzolanics for the f i r s t researches brickdust and pumice were used, and also tests were carried out with poor waterglass.
lOpm
Hore extensive researches were carried outh with suspensions of waterglass and pumice. By the one tests the suspension were mixed without, by the others they were mixed with tenside. The ratios of the components in the d i f f e r e n t mixes are shown in table 1. The great d i l u t i o n of the waterglass of normal commercial use ~ t h water (1 : 4 parts per weight) was chosen for reducing the thickness of the suspension f i l m .
Table 1: Ratio of the components in the suspensions (by weight) No.
water
waterglass
pumice
tenside
I 2
4 4
i 1
2 2
o o,16
3 4
4 4
i 1
3 3
o o,16
5
4
1
4
o
In some cases the cement mortar was placed on the aggregate, when the suspension film s t i l l was wet, in other cases the suspension f i l m dried before placing the cement mortar. In most of the cases the bond strength was greater at the samples with dried suspension f i l m than with wet suspension f i l m . In the following therefore only the researches with the dried suspension f i l m are described. The aggregates were polished pieces of granite. The storage of the samples and the execution of t h e ~ t e s t s were the same as described in (1).
Vol. 17, No. 4
657 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
In table 2 the values of bond strength are shown for the 5 d i f f e r e n t suspensions at various ages of hydration. In figure 7 these values are shown in comparison to the bond strength of the d i f f e r e n t aggregate materials without the use of a suspension. Table 2: Bond strength between aggregate (granite) and cement stone (w/c = o,35) as a r e s u l t of the use of waterglass - pumice tenside - suspension (suspension dried before placing cement mortar) No. Bond strength4 in N/mm ~7 at the age2~f hydration88ofdays cement in 1 2 3 4 5
1,3o 1,97 1,49 0,64 1,13
1,53 1,75 1,25 0,92 1,15
2,20 1,98 1,52 1,73 0,83
2,36 2,o6 1,72 2,27 2,05
In this diagram the great increase of bond strength by using the suspensions c l e a r l y can be seen. The course of the curves indicates a f u r t h e r increase of bond strength beyond the age of about Ioo days, especially for the samples with the suspensions Nr. i , 4 and 5. 3,0 2,6~
qudrt~ with suspension
2,4' Some samples with high values of bond strength showed fracture to some exent at the aggregate surface (as usual) and to the other in the cement stone. That means, that bond strength was at least in some parts of the contact zone greater than the strength of the cement stone, i t s e l f . Several samples with suspension Nr. 1 were stored in a long time t e s t for I year. The storage was carried out at about 99 % rel a t i v e humidity; on
~-~
/o/~
zz
~E2,o
/
z 1,8'
-"" ""
•:- 1,6"
"//
1,21 / /
~ :' ® A °j
/
I
"~®
plain aggregates o - ' l - - quarfz .... calcite
~D
0,6"
-'" ~
0,4"
.'"
0,2~
.-"
•
1
J ©
/°/
1,0 (3 "= 0,8[
O"
I No (D
2
~= ..~ .~ ~'_ ~: =~'-"
-- ~
feldspar gronife all polished
~-~'~
4
6 810 20 40 fime in days
60
80100
Figure 7: Strengthening of bond by covering the aggregates by the suspensions described above
658
Vol. 17, No. 4 R. Zimbelmann
the samples always water drops could be seen. A f t e r the time of 1 year the samples had a bond s t r e n g t h g r e a t e r than the s t r e n g t h of the cement. 12 of a series of 13 samples showed f r a c t u ~ t o t a l y in the cement stone, about lo to 15 mm above the contact zone. 1 sample f a i l e d at the aggregate surface, but in t h i s case the cement stone showed a great bubble e x a c t l y in t h i s area. Figure 8 shows an example of the t e s t e d samples.
Figure 8: Sample of p o r t l a n d cement stone on an aggregate, a f t e r bond t e s t at an age of 1 year. Bond s t r e n g t h exceeds the strength of cement stone.
Researches with steel In a s h o r t i n f o r m a t i v e series of researches i t should be tested wether the increase of bond s t r e n g t h could also be obtained at the surface of s t e e l . These researches were c a r r i e d out with the background t h a t in cases of f i r e concrete or mortar o f t e n is b u r s t i n g o f f from the cons t r u c t i o n over bars of concrete armation or of covered steel construct i o n s and the c o n s t r u c t i o n s thereby are exposed to a d d i t i o n a l danger. Or at l e a s t the bond s t r e n g t h between steel and concrete r a p i d l y is reduced by increasing temperatures as the researches of Sager and Rostasy (2) showed. Plain and ribbed bars of concrete steel with a diameter of 12 mm were cleaned from corrosion products by using a wire brush and then painted with the suspension lio. 1 (see t a b l e 1). A f t e r the suspension f i l m on the steel bars had d r i e d - the bars were stored in l a b o r a t o r y atmosphere-each bar was placed with one end in a mould of a 2o-cm-cube which then was f i l l e d with concrete. In the concrete age of 7 and 28 days pull out tests were c a r r i e d out. The compressive s t r e n g t h of concrete at the age of 7 days was 40 N/mm2 and 46 N/mmL at the age of 28 days. The p l a i n steel had a t e n s i l e s t r e n g t h of 34o N/mm2 and the ribbed steel had one of 5oo N/mm2. A f t e r c o n c r e t i n g the samples were stored 7 days in humide atmosphere (about 99 % r e l a t i v e humidity) and 21 days in a climate of 2o o C and 65 % r e l a t i v e humidity. The s l i p of the steel bars during the pull out t e s t was measured at the free end of the bar; see p i c t u r e 9. The r e s u l t s of the mull out t e s t s
Vol. 17, No. 4
659 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
20 cm
F i g u r e 9: Pull out t e s t , s c h e m a t i c a l l y shown
._-measurement of slip of the steel bar E u
i///~
# / / /
-
////>t
1
are shown in the p i c t u r e s lo and 11. They show a c l e a r i n c r e a s e in bond s t r e n g t h between the s t e e l bar and c o n c r e t e and even a f t e r the brake down o f the primary bond the further resistance against p u l l i n g out in a l l cases o f covered bars was h i g h e r than o f not covered bars. In t a b l e 3 the p u l l out s t r e s s e s a t the moment o f loss o f primary bond are compared.
'
z/,M~/ /
/ .r'q.'l z
.-
_%.//
0 ~ " " "-concrete cube
1, ~plastic ~
s
f
e
e
[
tube bar
I b = bond length=6,0cm 23
i
7 d o!9_s
22f
/- f
21-
/
/
28 days ---- "--" -I _d~ s _
/ /
28 days I / /
,,
/ ,I /
~E 3 E
/
z
/,,',,
.~2
rl I
I
7
,~
covered with suspension
.... uncovered
concrete compressive strengff ~ot on age of 7 do rs: [}cube : &2,7 Nlmm 2 28 days; [}cube : ~,0 Nlmm 2
0 0
0,2 0,6, 0,6 0,8 1,0 1,2 1,6, 1,6 1,8 2,0 sup of the free end in mm Figure I 0 : Shear s t r e n t h between p l a i n and covered s t e e l bars and concrete (steel unribbed)
o
£2 O,& 0,6 0,8 1,0 1,2 1,& 1,6 1,8 2,0 sup of the free end in mm
Figure 11: Shear s t r e n g t h between r i b b e d s t e e l bar and c o n c r e t e
660
Vol. 17, No. 4 R. Zimbelmann
Table 3: Pull out stresses at the loss of primary bond age of the samples in days
Pull out stresses in N/mm2 unribbed bars ribbed bars uncovered covered uncovered
covered
7
2,3
3,2
3,1
4,1
28
3,4
6,2
4,2
6,6
Summary On the basis of the knowledge about the structure of the contact zone between aggregate and cement stone the reason for this specific formation of structure is discussed. By this point of view i t was t r i e d to influence the contact zone during i t s phase of formation in such a manner to get the structure growing denser and increasing the physical adhesion to the aggregate. This idea succeeded in to ways. In one way by using a detergent. By this the thickness of the contact zone and the porosity of the i n t e r mediate layer was reduced, the bond strength increased. On the other way the increase of the bond strength was reached by covering the aggregate with a suspension of a pozzolanic material in water glass. Thereby the calcium hydroxide was bound to the pozzolanic material (pumice) which was fastened to the aggregate surface by the aid of the water glass. The water glass had an additional effect by the fact that i t s product of decomposition, SiO 2, effected a f u r t h e r increase of density of the contact zone. By this the bond strength was increased in a great measure. The increase of bond strength at the age of one year was so high, that the bond strength exceeded the tensile strength of cement stone. In pull out tests also an increase of bond strength between steel bars and concrete was confirmed when the steel bars had been covered with the above mentioned suspension. References I. R. Zimbelmann, A Contribution to the Problem of Cement-Aggregate Bond, Cem. Concr. Res. 15, 8ol - 8o8 (1985) 2. H. Sager and F.S. Rost6sy, The Effect of Elevated Temperature on the Bond Behaviour of Embedded Reinforcing Bars, Bond and Concrete, Proc. of the Intern. Conf. on Bond in Concrete, 1982, 2o6 - 216
A METHOD FOR STRENGTHENING THE BOND BETWEEN CEMENT STONE AND AGGREGATES R. Zimbelmann U n i v e r s i t ~ t der Bundeswehr MUnchen Neubiberg, F. R. Germany
(Communicated by F.W. Locher) (Received March 25, 1987) ABSTRACT On the basis of the knowledge about the contact zone between aggregate and cement stone researches were c a r r i e d out with the aim to enlarge the bond strength of these components. The researches lead to an increase of bond strength to such an a l t i t u d e , t h a t at the age of one year the t e n s i l e strength of cement stone was exceeded. For t h i s purpose the aggregate surface had been covered by a suspension of pumice, detergent and water glass. In pull out t e s ~ w i t h steel bars, which were covered with the suspension described above a remarcable increase of bond i n concrete was obtained. Introduction In (1) the author described the contact zone between aggregate surface and cement stone. According to t h i s cement stone does not surround the aggregate grains in concrete in a homogenous mass, but a system of d i f f e r e n t layers i s formed. The contact zone, cons i s t i n g of the contact layer and the intermediate l a y e r , to a predominant part consists of calcium hydroxide (Ca(OH)2). By t h i s material also the connexion (bond) of cement stone to the aggregate is set up. This area does represent the weakest zone i n concrete s t r u c t u r e exposed to compression, tens:ion or shear stresses. In f i g u r e 1 the d e s t r i b u t i o n of stresses arround the aggregate i n concrete loaded by compression is shown.
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
/i:i!i:.;:11;i;!;!i!i:i!;:.i!;2:::i:'::.:i:i::::.::. (cement stone ]
i!iii!!ii
!iiiiiii!;)iiiiiiii!iii!?iiiiiiiiiiiiiii
Figure 1 : D i s t r i b u t i o n of stresses arround a compression loaded aggregate 651
652
Vol. 17, No. 4 R. Zimbelmann
An increase of the strength properties of concrete therefore should to be reached by strengthening the contact zone, whatever one could understand of "strengthening". The following t r e a t i s e shall describe the thoughts and experiments which led to a remarkable increase in bond strength between cement stone and aggregate. Formation of the contact zone The basis for the researches on the modification and strengthening of the contact zone had to be the knowledge upon the reason of i t s f o r mation, expecially the concentration of the Ca(OH)2 and the high porosity of this zone, reaching up to a percentage of cavity of about 5o %. These blo symptoms are a sign of a r e l a t i v e l y hlgh water content existing in this area during and a f t e r bringing toaether the components aggregate and cement mortar. This supposition also is sustained by confirming that no cement grains could be found in the area of the contact zone in a thickness of about 15 to 25 um. This led to the conclusion that during mixing and bringing cement mortar on the aggregate a water f i l m is formed arround each solid p a r t i c l e . The thickness of the water film may be about half of the thickness of the contact zone, i t thereafter may reach about io pm. This is schematically shown in figure 2. !
ticles
Figure 2: Formation of the water film during bringing together cement mortar and aggregate
I
wnfer film
i.n
oggregate After bringing water to the solid p a r t i c l e s , these are covered at once by the water f i l m , t~hich posesses nearly a constant thickness at a l l p a r t i c l e s . In consequence of this water film the p a r t i c l e s are separated from each other at a distance of about twice of the thickness of water film. At the beginning of hydration of the cement calciumhydroxide and the components, which l a t e r w i l l form e t t r i n g i t e are dissolved in the mixing water arround the cement grains and diffuse into the vlater f i l m arround
Vol. 17, No. 4
653 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
the i n e r t aggregate. Here at the i n e r t aggregate surface the dissolved substances can c r i s t a l l y s e in a qreater extent than at the periphery of the cement grains, which permanently is changing. The contact layer is formed. I t can be presumed that by the accumulation of crystals at the aggregate surface and the growth of the crystals the water film may be expelled and thereby at least a part of the expelled water may extent the distance from the aggregate surface to the nearest situated cement grains. During hydration a dense gel is formed arround and between the cement grains, which is growing from both sides of the adjacent cement grains. At the cement grains adjoining the i n e r t aggregate the hydrates only emanate from one of the two partners, the cement grain. The water film arround the aggregate therefore is bridged by some great crystals finding here much space for growing. Thougths about strengthening the adhesion Decisive for the formation of the contact zone and therefore for the low bond strength the two following factors may be seen: a) formation of a water film arround the aggregate b) c r i s t a l l i s a t i o n of calcium hydroxide in form of the contact layer. Starting from this thought t h e o r e t i c a l l y there can be expected an augmentation of bond in consequence of the following measures. F i r s t p o s s i b i l i t y : By using tensides the surface tension of water is reduced. The'consequence out of this is a diminishing of the water f i l m , thereby a greater density of the contact zone and an enlarging number of contact points between the contact layer and the intermediate layer may be reached. So the area to transmit the stresses could be enlarged. Second p o s s i b i l i t y : I f there could be induced a chemical or a physical reaction between the aggregate and the cement stone - as i t can be supposed of the lime stone aggregate - a t i g h t connection over the aggregate surface could be reached. Third p o s s i b i l i t y : A combination of the p o s s i b i l i t i e s 1 and 2 could strengthen the effect of bonding because of the diminished water film out of the use of a tenside the partners of the reaction can move closer. Fourth p o s s i b i l i t y : I f one deposits reactive substances at the aggregate surface i t might be possible to receive reaction products out of this substance and the calciumhydroxid, which have a greater physical a f f i n i t y to the aggregate - in most of the cases to q u a r t z i t i c aggregates - and at the same time gain a greater density of the contact zone. The reactive substance would then be a bridge between the aggregate surface and the cement stone. Experiments about the strengthenin~ of the adhesion In a short test the f i r s t variant of the 4 p o s s i b i l i t i e s was tested. In an extensive research the fourth variant was examined. In the tests with a tenside, a small quantity of tenside was added to the mixing water. The tenside consisted of a washing agent of normal commercial usage. The water with the added agent (about 1 to 3 % of cement weight) was slowly mixed together with a portland cement PZ 35 F
654
Vol. 17, No. 4 R. Zimbelmann
in a laboratory mixer so that no foam was formed. The water-cementr a t i o by weight was o.35. The exposition auf the cement on the aggregate and the performance of the tests were carried out as described in ( I ) . The t e s t i n g of the bond at only a few samples of polished lime stone, quarz gravel and polished feldspar showed an increase of the bond strength of about Ioo to 18o per cent in r e l a t i o n to the values of bond strength reached with the same aggregates and the same cement but without the use of a tenside. The r e s u l t s of these f i r s t tests are shown in figure 3.
3,0 2,8 2,6 ~E
(28o%) I
2,4
E 2,2
z 2,o .~ 1,8r-
I
1,6
cm 1,z,
t'--
1,2 •4.- 1,0
t
(200%)
..~ 0,8 c 0,6 o •. ~ O,L, 0,2
p-
~s
2
;
/
z limestone, polished
/
__
q ucLrtz, grave[ fe[clspar, polished
f
~...~~. ~ 2
~
6
~
81o 20 co 60 80100 time in days
Figure 3: F i r s t tests of strengthening the adhesion between cement mortar and aggregate by adding a tenside to the mixing water
An examination of the contact zone by the scanning electron microscope showed t h a t the contact zone was - as expected - s i s n i f i c a n t l y thinner and of higher densi~,. The contact layer had a thickness of about o,5 to i ~m insted of 2 to 3 um. The region of the intermediate zone showed a changed s t r u c t u r e in comparison to cement stone without tenside. Besides of the well known form of the calcium hydroxide c r i s t a l l s (small sheets and great panel shaped c r i s t a l l s ) the s t r u c t u r e was s o l i d with some greater pores. The s t r u c t u ~ , rich of pores, consisting of a texture of needle c r i s t a l l s out of e t t r i n g i t e and CSH mostly was not to be identifyed. Moreover there was remarkable t h a t in spite of the humid storage already at a r e l a t i v e l y ~oung age (28 days) the contact layer showed numerous microcracks. With increasing age needleshaped CSH-crystals formed in the pores of the intermediate layer.
Vol. 17, No. 4
655 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
As s i g h t photographs of the aggregate surface showed a f t e r the bond t e s t , the area of the contact l a y e r , which had been losened by the bond t e s t , was greater than in the t e s t w i t h o u t the use of tenside. That means, t h a t the linkage between the contact l a y e r and cement stone had been increased and to t h i s the enhancement of the bond strength may be r e l a t e d . Figure 4 shows the contact l a y e r a f t e r the bond t e s t . The numerous cracks are c l e a r l y to be seen. Figure 5 shows an area of the intermediate layer w i t h o u t the use of tenside and f i g u r e 6 with the use of tenside. d ~
I
&O~m
Figure 4: Contact layer a f t e r the use of a tenside
The t e s t according to the p o s s i b i l i t y 4 based on the f o l l o w i n g idea. I f i t was possible to replace the contact layer of calcium hydroxide by a layer of calcium s i l i c a t e s , i t could be expected, t h a t , e s p e c i a l l y in those cases the aggregate would be of quartz or would contain parts of quartz, there would e x i s t a higher physical a f f i n i t y between t h i s s i l i c a t e contact layer and the aggregate surface. Besides of t h i s a b e t t e r connection between the s i l i c a t e contact l a y e r and the hydrates of the cement could be expected. F o r t h e case t h a t t h i s s i l i c a t e layer does cons i s t of a pozzolanic m a t e r i a l , the calcium hydroxide may react with i t by forming CSH. So the formation of a l a y e r or of greater comprisals c o n s i s t i n g of calcium hydroxide might be avoided.
10 .,}]m Figure 5:Intermediate layer w i t h o u t the use o f a tenside
On the bases of these thoughts several suspensions out of waterglass (Potasium s i l i c a t e and sodium s i l i c a t e ) and
656
Vol. 17, No. 4 R. Zimbelmann
pozzolanics were mixed. The waterglass on the one side had to fasten the pozzolanic on the aggregate surface and on the other side i t had to give a greater density to the contact zone - especially to the indermediate layer by i t s products of decomposition in the reaction with the calcium hydroxide. F i r s t researches showed that the effect of such a suspension was the b e t t e r , the thinner the film out of this suspensi£n on Figure 6: Intermediate layer a f t e r the the aggregate surface use of a tenside could be formed. As pozzolanics for the f i r s t researches brickdust and pumice were used, and also tests were carried out with poor waterglass.
lOpm
Hore extensive researches were carried outh with suspensions of waterglass and pumice. By the one tests the suspension were mixed without, by the others they were mixed with tenside. The ratios of the components in the d i f f e r e n t mixes are shown in table 1. The great d i l u t i o n of the waterglass of normal commercial use ~ t h water (1 : 4 parts per weight) was chosen for reducing the thickness of the suspension f i l m .
Table 1: Ratio of the components in the suspensions (by weight) No.
water
waterglass
pumice
tenside
I 2
4 4
i 1
2 2
o o,16
3 4
4 4
i 1
3 3
o o,16
5
4
1
4
o
In some cases the cement mortar was placed on the aggregate, when the suspension film s t i l l was wet, in other cases the suspension f i l m dried before placing the cement mortar. In most of the cases the bond strength was greater at the samples with dried suspension f i l m than with wet suspension f i l m . In the following therefore only the researches with the dried suspension f i l m are described. The aggregates were polished pieces of granite. The storage of the samples and the execution of t h e ~ t e s t s were the same as described in (1).
Vol. 17, No. 4
657 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
In table 2 the values of bond strength are shown for the 5 d i f f e r e n t suspensions at various ages of hydration. In figure 7 these values are shown in comparison to the bond strength of the d i f f e r e n t aggregate materials without the use of a suspension. Table 2: Bond strength between aggregate (granite) and cement stone (w/c = o,35) as a r e s u l t of the use of waterglass - pumice tenside - suspension (suspension dried before placing cement mortar) No. Bond strength4 in N/mm ~7 at the age2~f hydration88ofdays cement in 1 2 3 4 5
1,3o 1,97 1,49 0,64 1,13
1,53 1,75 1,25 0,92 1,15
2,20 1,98 1,52 1,73 0,83
2,36 2,o6 1,72 2,27 2,05
In this diagram the great increase of bond strength by using the suspensions c l e a r l y can be seen. The course of the curves indicates a f u r t h e r increase of bond strength beyond the age of about Ioo days, especially for the samples with the suspensions Nr. i , 4 and 5. 3,0 2,6~
qudrt~ with suspension
2,4' Some samples with high values of bond strength showed fracture to some exent at the aggregate surface (as usual) and to the other in the cement stone. That means, that bond strength was at least in some parts of the contact zone greater than the strength of the cement stone, i t s e l f . Several samples with suspension Nr. 1 were stored in a long time t e s t for I year. The storage was carried out at about 99 % rel a t i v e humidity; on
~-~
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zz
~E2,o
/
z 1,8'
-"" ""
•:- 1,6"
"//
1,21 / /
~ :' ® A °j
/
I
"~®
plain aggregates o - ' l - - quarfz .... calcite
~D
0,6"
-'" ~
0,4"
.'"
0,2~
.-"
•
1
J ©
/°/
1,0 (3 "= 0,8[
O"
I No (D
2
~= ..~ .~ ~'_ ~: =~'-"
-- ~
feldspar gronife all polished
~-~'~
4
6 810 20 40 fime in days
60
80100
Figure 7: Strengthening of bond by covering the aggregates by the suspensions described above
658
Vol. 17, No. 4 R. Zimbelmann
the samples always water drops could be seen. A f t e r the time of 1 year the samples had a bond s t r e n g t h g r e a t e r than the s t r e n g t h of the cement. 12 of a series of 13 samples showed f r a c t u ~ t o t a l y in the cement stone, about lo to 15 mm above the contact zone. 1 sample f a i l e d at the aggregate surface, but in t h i s case the cement stone showed a great bubble e x a c t l y in t h i s area. Figure 8 shows an example of the t e s t e d samples.
Figure 8: Sample of p o r t l a n d cement stone on an aggregate, a f t e r bond t e s t at an age of 1 year. Bond s t r e n g t h exceeds the strength of cement stone.
Researches with steel In a s h o r t i n f o r m a t i v e series of researches i t should be tested wether the increase of bond s t r e n g t h could also be obtained at the surface of s t e e l . These researches were c a r r i e d out with the background t h a t in cases of f i r e concrete or mortar o f t e n is b u r s t i n g o f f from the cons t r u c t i o n over bars of concrete armation or of covered steel construct i o n s and the c o n s t r u c t i o n s thereby are exposed to a d d i t i o n a l danger. Or at l e a s t the bond s t r e n g t h between steel and concrete r a p i d l y is reduced by increasing temperatures as the researches of Sager and Rostasy (2) showed. Plain and ribbed bars of concrete steel with a diameter of 12 mm were cleaned from corrosion products by using a wire brush and then painted with the suspension lio. 1 (see t a b l e 1). A f t e r the suspension f i l m on the steel bars had d r i e d - the bars were stored in l a b o r a t o r y atmosphere-each bar was placed with one end in a mould of a 2o-cm-cube which then was f i l l e d with concrete. In the concrete age of 7 and 28 days pull out tests were c a r r i e d out. The compressive s t r e n g t h of concrete at the age of 7 days was 40 N/mm2 and 46 N/mmL at the age of 28 days. The p l a i n steel had a t e n s i l e s t r e n g t h of 34o N/mm2 and the ribbed steel had one of 5oo N/mm2. A f t e r c o n c r e t i n g the samples were stored 7 days in humide atmosphere (about 99 % r e l a t i v e humidity) and 21 days in a climate of 2o o C and 65 % r e l a t i v e humidity. The s l i p of the steel bars during the pull out t e s t was measured at the free end of the bar; see p i c t u r e 9. The r e s u l t s of the mull out t e s t s
Vol. 17, No. 4
659 CEMENT PASTE, AGGREGATE, BOND, STRENGTHENING
20 cm
F i g u r e 9: Pull out t e s t , s c h e m a t i c a l l y shown
._-measurement of slip of the steel bar E u
i///~
# / / /
-
////>t
1
are shown in the p i c t u r e s lo and 11. They show a c l e a r i n c r e a s e in bond s t r e n g t h between the s t e e l bar and c o n c r e t e and even a f t e r the brake down o f the primary bond the further resistance against p u l l i n g out in a l l cases o f covered bars was h i g h e r than o f not covered bars. In t a b l e 3 the p u l l out s t r e s s e s a t the moment o f loss o f primary bond are compared.
'
z/,M~/ /
/ .r'q.'l z
.-
_%.//
0 ~ " " "-concrete cube
1, ~plastic ~
s
f
e
e
[
tube bar
I b = bond length=6,0cm 23
i
7 d o!9_s
22f
/- f
21-
/
/
28 days ---- "--" -I _d~ s _
/ /
28 days I / /
,,
/ ,I /
~E 3 E
/
z
/,,',,
.~2
rl I
I
7
,~
covered with suspension
.... uncovered
concrete compressive strengff ~ot on age of 7 do rs: [}cube : &2,7 Nlmm 2 28 days; [}cube : ~,0 Nlmm 2
0 0
0,2 0,6, 0,6 0,8 1,0 1,2 1,6, 1,6 1,8 2,0 sup of the free end in mm Figure I 0 : Shear s t r e n t h between p l a i n and covered s t e e l bars and concrete (steel unribbed)
o
£2 O,& 0,6 0,8 1,0 1,2 1,& 1,6 1,8 2,0 sup of the free end in mm
Figure 11: Shear s t r e n g t h between r i b b e d s t e e l bar and c o n c r e t e
660
Vol. 17, No. 4 R. Zimbelmann
Table 3: Pull out stresses at the loss of primary bond age of the samples in days
Pull out stresses in N/mm2 unribbed bars ribbed bars uncovered covered uncovered
covered
7
2,3
3,2
3,1
4,1
28
3,4
6,2
4,2
6,6
Summary On the basis of the knowledge about the structure of the contact zone between aggregate and cement stone the reason for this specific formation of structure is discussed. By this point of view i t was t r i e d to influence the contact zone during i t s phase of formation in such a manner to get the structure growing denser and increasing the physical adhesion to the aggregate. This idea succeeded in to ways. In one way by using a detergent. By this the thickness of the contact zone and the porosity of the i n t e r mediate layer was reduced, the bond strength increased. On the other way the increase of the bond strength was reached by covering the aggregate with a suspension of a pozzolanic material in water glass. Thereby the calcium hydroxide was bound to the pozzolanic material (pumice) which was fastened to the aggregate surface by the aid of the water glass. The water glass had an additional effect by the fact that i t s product of decomposition, SiO 2, effected a f u r t h e r increase of density of the contact zone. By this the bond strength was increased in a great measure. The increase of bond strength at the age of one year was so high, that the bond strength exceeded the tensile strength of cement stone. In pull out tests also an increase of bond strength between steel bars and concrete was confirmed when the steel bars had been covered with the above mentioned suspension. References I. R. Zimbelmann, A Contribution to the Problem of Cement-Aggregate Bond, Cem. Concr. Res. 15, 8ol - 8o8 (1985) 2. H. Sager and F.S. Rost6sy, The Effect of Elevated Temperature on the Bond Behaviour of Embedded Reinforcing Bars, Bond and Concrete, Proc. of the Intern. Conf. on Bond in Concrete, 1982, 2o6 - 216